Generally, on machine tools, a spindle is rotatably supported inside a housing through a plurality of bearings, and that spindle can be rotated by actuating a motor. Thus, as such a machine tool starts to operate, the motor is rotationally actuated and the bearings are rotated, so that they generate heat themselves. The heat generated in them is then transmitted to the spindle, thereby causing thermal deformation (thermal expansion) of the spindle. When such thermal deformation of the spindle occurs, the position of the edges of the tool mounted to this spindle is displaced, which greatly affects the machining accuracy.
In view of this, machine tools have heretofore been provided with a spindle cooling device for cooling their spindles. This spindle cooling device supplies and circulates cooling oil around the spindle to cool the spindle, and also causes the cooling oil in the middle of the circulation to exchange heat with coolant gas to maintain the temperature of that cooling oil at a predetermined temperature. A spindle cooling device for a machine tool as above is disclosed in Patent Document 1, for example.